Study of axonal dystrophy. III. Posterior funiculus and posterior column of ageing and old rats

Brain and spinal cord lesions in 28 inbred strains of aging mice

Brain and spinal cord histopathology findings in male and female 20-month-old mice in a large-scale aging study of 28 inbred Jackson Laboratory mouse strains from 7 genetic families are described. Brain sections from selected strains at 12 and 24 months of age or older were also reviewed. Common lesions include axonal dystrophy in the gracile and/or cuneate nucleus in the sensory tract of the dorsal medulla and in the spinal cord in all strains. Hirano-like bodies were seen in 24/28 strains, and mineralization was observed in the thalamus of 9/28 strains. Less common lesions were also seen in the cerebellum, cerebral cortex, and other brain areas. No brain or spinal cord tumors were found. Evidence of an impairment of the ubiquitin-proteasome system (UPS) and/or suspected autophagy was manifested as medullary axonal dystrophy with intra-axonal granular eosinophilic bodies and LC3B immunohistochemistry in most strains. RIIIS/J, the most severely affected strain, showed moderate axonal dystrophy at 12 months, which progressed to severe lesions at 20 months. Comparative pathology in various species is discussed.

Disruption of fibroblast growth factor receptor signaling in nonmyelinating Schwann cells causes sensory axonal neuropathy and impairment of thermal pain sensitivity

Axon-glial interactions are critical for normal functioning of peripheral nerves, and their disruption leads to peripheral neuropathies. Fibroblast growth factors (FGFs) are key players in peripheral nerve regeneration after injury. We investigated the role of FGF receptor (Fgfr) signaling in Schwann cells and the consequent regulation of normal Schwann cell-axon interactions. Fgfr1 and Fgfr2 were conditionally inactivated, either singly or in combination, in myelinating and nonmyelinating Schwann cells (NMSCs) of transgenic mice. The double mutant mice displayed significant loss of thermal sensitivity accompanied by marked neuropathy of unmyelinated nociceptive sensory axons terminating in the dorsal horn of spinal cords, the primary site for integrating pain and temperature inputs. Neuropathy, although to a lesser extent, was also observed in the nociceptive C-fibers in the Remak bundles of sciatic nerves; however, there was no loss of NMSCs that ensheathe these axons. Furthermore, axons wrapped by myelinating Schwann cells and associated myelin sheaths appeared to be unaffected. Relative to the double mutants, axonal neuropathy developed much later in the Fgfr1 but not Fgfr2 single mutant, indicating a difference in signaling potential of the two receptors, with Fgfr1 being more robust than Fgfr2. These findings emphasize the importance of Fgfr1 and Fgfr2 signaling as potential mediators of axon-glial interaction in the peripheral sensory pain pathway primarily via influencing NMSC function, which in turn modulates the structure and function of unmyelinated sensory axons. This study provides a novel molecular mechanism for nociception with possible implications for pain sensitivity in peripheral sensory neuropathies.

Encephalomyelitis, brain tumors, neuromuscular diseases and miscellaneous disorders

Japanese neuropathologists have accomplished and contributed to a considerable number of achievements, and some of these are cited in other articles in this issue. Several of these achievements as well as other miscellaneous discoveries are briefly summarized in the present paper. Specifically these relate to rabies postvaccinal encephalomyelitis, experimental allergic encephalomyelitis, brain tumor research, neuromuscular disorders, schizophrenia, viral infections, glial inclusion body in multiple system atrophy, and the neurobiology of glia.

Regulation of neurotrophin signaling in aging sensory and motoneurons: dissipation of target support?

A hallmark of senescence is sensorimotor impairment, involving locomotion and postural control as well as fine-tuned movements. Sensory and motoneurons are not lost to any significant degree with advancing age, but do show characteristic changes in gene-expression pattern, morphology, and connectivity. This review covers recent experimental findings corroborating that alterations in trophic signaling may induce several of the phenotypic changes seen in primary sensory and motoneurons during aging. Furthermore, the data suggests that target failure, and/or breakdown of neuron-target interaction, is a critical event in the aging process of sensory and motoneurons.

Upregulation of GFRalpha-1 and c-ret in primary sensory neurons and spinal motoneurons of aged rats

Aging is associated with a decline in neuromuscular and somatosensory functions. Senile muscle atrophy, considered to be of neurogenic origin, is prevalent, and sensory thresholds increase with age. However, the loss of motoneurons and primary sensory neurons is small, while sensory and motor innervation appears disturbed due to aging-related axon lesions. One mechanism which may play a role in this process is altered trophin signaling. We here report that the glial cell line-derived neurotrophic factor (GDNF) receptor GFRalpha-1 mRNA and GFRalpha-1 protein-like immunoreactivity are upregulated in spinal motoneurons, and in dorsal root ganglion neurons of 30-month-old rats. The established signaling mechanism for the GDNF/GFRalpha-1 complex is through binding to the tyrosine kinase receptor encoded by the c-ret proto-oncogene, and we also show here that c-ret mRNA is upregulated in both motoneurons and primary sensory neurons of aged rats. The findings reported here, combined with evidence presented in other studies of changes in p75(NTR) and trk receptor expressions in aging primary sensory neurons and motoneurons, point at marked alterations in trophin signaling in senescence.

Expression of p75(NTR), trkB and trkC in nonmanipulated and axotomized motoneurons of aged rats

Several lines of evidence indicate that adult neurons remain dependent on neurotrophins and that changes in tissue expression of neurotrophins and/or their receptors may play a role in senile neurodegeneration. We have studied the expression of p75NTR, trkB and trkC, respectively, in lumbar motoneurons of young adult (2-3 months) and aged (30 months) rats subjected to sciatic transection using in situ hybridization and immunohistochemistry. Nonmanipulated age-matched animals were processed in parallel. In nonmanipulated aged rats, high levels of p75NTR could be seen in a number of motoneurons (10-15%), while in young adult animals no p75NTR could be detected. Seven days following sciatic axotomy, a conspicuous ipsilateral upregulation p75NTR was observed in young adult rats. Also in aged rats there was a marked ipsilateral increase in number of p75NTR expressing neurons ( approximately 100%). In comparison to young adult rats, aged rats showed a decreased expression of both trkB (5/6 animals) and trkC (6/6 animals). Furthermore, in response to sciatic transection, 3 out of 5 aged rats did not show an increased expression of trkB. In aged rats, axotomy did not induce any significant change in trkC expression. In the young adult rats, we recorded a side-to-side effect with lower values ipsilaterally, however, it cannot be excluded that this difference was caused by an upregulation in the contralateral motoneurons. Oligonucleotide probes against BDNF and NT3 mRNA showed only very few faintly positive neurons in both age groups. Our results indicate that the pattern of regulatory changes of NT receptors in response to axotomy is different in aged and young adult rats. The lack of covariation between p75NTR and trkB and trkC regulation in aged rats indicates a changed role for p75NTR in senescent motoneurons.

Neuropeptides and neurotrophin receptor mRNAs in primary sensory neurons of aged rats

Neuropeptides and neurotrophin receptors are regulated in primary sensory neurons in response to axonal injury, and axonal lesions are characteristic stigmata of aging primary sensory neurons. We have therefore examined the expression of neuropeptides and neurotrophin receptor mRNAs in 30-month-old (median survival age) Sprague-Dawley rats to see if similar adaptive mechanisms operate in senescence. The content of neuropeptides was examined with immunohistochemistry (IHC) and in situ hybridization (ISH), and the cellular mRNA expression of neurotrophin receptors was studied with ISH. All of the aged rats had symptoms of hind limb incapacity (posterior paralysis), but fore limbs did not seem affected. The size-distribution of neuronal profiles in cervical and lumbar dorsal root ganglia (DRGs) was similar in aged and young adult (2-3 months old) rats. In aged rats, the DRG neurons showed an increase in both immunolabelling and mRNA content of neuropeptide tyrosine (NPY), as well as an increased cellular expression of galanin mRNA. In the same animals, there were decreased cellular levels of calcitonin gene-related peptide (CGRP; IHC and ISH) and substance P (SP; IHC and ISH), while the difference in neuronal somatostatin (IHC and ISH) was small. The distribution of neuropeptide immunoreactivities in the dorsal horn of the corresponding spinal cord segments revealed a decreased labelling for CGRP-, SP-, and somatostatin-like immunoreactivities (LI) in the aged rats at both cervical and lumbar levels. NPY- and galanin-LI had a similar distribution in aged and young adult rats. NPY-immunoreactive fibers were also encountered in the dorsal column of aged but not young adult rats. ISH revealed that most of the primary sensory neurons express mRNA for the p75 low-affinity neurotrophin receptor (p75-LANR) and that there was no discernible difference between young adult and aged rats. The labelling intensity for mRNA encoding high-affinity tyrosine kinase receptors (TrkA, TrkB, and TrkC) was decreased in aged rat DRG neurons, while the percentage of neuronal profiles expressing mRNA for TrkA/B/C was similar in young adult and aged rats. The changed pattern of neuropeptide expression in primary sensory neurons of aged rats resembled that seen in young adult rats subjected to axonal injury of peripheral sensory nerves and may, thus, indicate aging-related lesions of sensory fibers. Since NPY is primarily present in large and galanin in small DRG neurons, the stronger effect on NPY as compared to galanin expression may indicate that aging preferentially affects neurons associated with mechanoreception (A alpha and A beta fibers) as compared to nociceptive units (A delta and C fibers). Furthermore, the observed changes in neuropeptide expression were most pronounced in lumbar DRGs, that harbors the sensory neurons supplying the affected hindlimbs of the rats.

Calcitonin gene-related peptide- and substance P-immunoreactive axons in the nucleus gracilis of the rat with special reference to axonal dystrophy: light and electron microscopic observations

Calcitonin gene-related peptide (CGRP) and substance P (SP)-immunoreactive (IR) axons in the nucleus gracilis of normal rats (1-15 months of age) were studied by light and electron microscopy. Besides many CGRP-IR and SP-IR varicosities with normal appearance, we found a few swollen (nearly round or oval) varicosities with either CGRP or SP immunoreactivity. Swollen CGRP-IR varicosities were more frequently seen than SP-IR ones, appearing from 3 months of age and increasing in number and size (up to approximately 25 microns in diameter) with advancing age. At the electron microscopic (EM) level, CGRP-IR and SP-IR swollen varicosities showed dystrophic changes, i.e., many membranous dense bodies, and proliferation of microtubules and neurofilaments. CGRP-IR or SP-IR dystrophic axons also contained many mitochondria and sometimes made synaptic contacts with nonreactive dendrites (occasionally with non-IR axons). These findings suggest that the dystrophic CGRP and SP axonal profiles represent a functionally distinct subpopulation of axonal dystrophy in the nucleus gracilis and use CGRP or SP as a neuroactive substance. Using a double-immunostaining method, many of normal CGRP-IR axons were identified to be SP-IR. However, no single dystrophic varicosity was found to contain both CGRP and SP immunoreactivities. These findings suggest that CGRP and SP afferents are independently affected and progress to dystrophic changes.

Complement and clusterin in the spinal cord dorsal horn and gracile nucleus following sciatic nerve injury in the adult rat

We provide evidence for activation of the complement cascade in the dorsal horn of the spinal cord and in the gracile nucleus in the brainstem following sciatic nerve transection in the adult rat. Immunocytochemical analyses showed immunoreactivity for endogenous immunoglobulin G as shown by immunostaining with F(ab')2 antibodies, as well as complement factors C1, C1q, C3, C3d and C9 in the appropriate central termination areas of the injured sciatic nerve. Results from double labelling immunocytochemistry showed a strong association between immunoglobulin and complement factors on the one hand and reactive microglia on the other. However, some complement immunoreactivity was also found in the neuropil, possibly representing secreted complement. In situ hybridization with an oligonucleotide probe showed a marked increase in C3 messenger RNA, indicating local synthesis of C3 protein. In parallel with activation of complement, there was an increased immunoreactivity for the putative complement inhibitor clusterin, which co-localized with glial fibrillary acidic protein-positive astrocytes. In situ hybridization showed an increased labelling of clusterin messenger RNA. These findings indicate that complement activation and up-regulation of complement inhibitors are prominent central responses to peripheral sensory nerve injury. These responses may therefore be important elements underlying so-called transganglionic degenerative changes in primary sensory axons and terminals.

Fine structural alteration in target-deprived axonal terminals in the rat thalamus

Lesioning of thalamic target neurons in the adult rat provokes a resorption of terminal axonal branches of afferent neurons from the dorsal column nuclei. There is, however, no massive neuronal loss in the dorsal column nuclei. In the adult, therefore, the thalamic post synaptic target cells influence primarily the extent of the terminal component of the afferent neurons. The subcellular changes underlying the regression of these adult terminals are unknown. To address this issue, we have looked at the electron microscopic level for the ultrastructural correlates of this retraction of the terminal compartment of target-deprived neurons in the adult rat thalamus. By analysing the fine structure of target-deprived axons and their immunoreactivity for a specific synaptic protein, synaptophysin, we have observed that all the organelles and the protein formed in the cell body, including dense-core vesicles, continue to be transported to the terminal compartment and accumulate at this level. At the terminal level, engorgement of organelles induces the formation of varicosities. An enormous increase in local degradative activity occurs in parallel to this accumulation. In contrast, organelles involved in membrane turnover and degradation (including synaptic and coated vesicles, multivesicular bodies, lysosomes) in the nerve terminals are clearly modified. There is a progressive loss of synaptic vesicles, whereas clathrin-coated vesicles and multivesicular bodies are numerous. We propose that the resorption of terminal axonal branches after thalamic target deprivation in the adult is associated with a bias of the system of membrane recycling at the axonal terminals towards degradation. In the absence of apparent changes in the pathways originating from the cell body in these conditions, it is unlikely that presynaptic neurons in the adult thalamus are dependent upon post synaptic target neurons for the delivery of organelles and proteins to the terminals.

Altered antigen expression of microglia in the aged rodent CNS

Microglia, the resident macrophages of the central nervous system, are characterised by a highly specialized morphology and unusual antigenic phenotype. Microglia appear to be downregulated by their microenvironment when compared to other tissue macrophages. We have studied the microglia in brains of healthy, aged rats with a panel of monoclonal antibodies. We have found that microglia in the brains of these aged rats express antigens that are downregulated or absent from microglia of juvenile rats. The stimuli which give rise to this upregulated phenotype are not known. Age related changes in the phenotype of microglia should be taken into account when considering the possible role of microglia in neuropathological conditions.

Ultrastructural changes in the gracile nucleus of the rat after sciatic nerve transection

Ultrastructural changes in the gracile nucleus of the rat have been examined after peripheral nerve injury. The sciatic nerve of adult rats was transected at mid-thigh level, and after survival times ranging from 1 day to 32 weeks sections from the gracile nucleus were prepared for electron microscopic examination. Unoperated animals served as controls. Atypical profiles were regularly observed in the experimental cases at post-operative survival times from 3 days up to 32 weeks. It was sometimes not possible to classify these as preterminal axons or terminals, because synaptic contacts could not be identified. The two most common changes throughout the entire post-operative period were greatly expanded myelinated axons, or unmyelinated profiles containing numerous mitochondria, osmiophilic dense bodies and vacuoles. Atypical profiles were occasionally observed in unoperated control animals. The results clearly show that various types of degenerative changes occur in the gracile nucleus after peripheral nerve injury. These changes differ markedly from previously described transganglionic changes in other systems. It cannot be excluded that some of the changes reflect growth-related reactions, although the typical features of axon regeneration could not be found.